In known apparatus for the separation of solid substances from a stream of gas, there has been a problem with the formation of deposits of particles on the surfaces of deflecting plates and in the collection grooves formed therein. Particles of material present in the stream of a crude gas being purified tend to adhere to the surfaces associated with the deflecting plates. Such a particle deposition is disadvantageous to particle separation. The prior art apparatus must, therefore, from time to time be taken out of services in order to free the deflecting plates and the collecting grooves arranged in end areas thereof from formations of deposited particles. Such procedure, however, is connected with great expenditures of work, time, and costs.